1. Technical Field
The present invention relates to a MEMS structure and a manufacturing method thereof, more particularly, to an optical modulator and a manufacturing method thereof.
2. Description of the Related Art
MEMS refers to a microelectromechanical system or element, which is a technology that uses semiconductor manufacturing technology to form three-dimensional structures on silicon substrates. There are a variety of applications in which MEMS is used, an example of which is the field of optics. Using MEMS technology allows the manufacture of optical components smaller than 1 mm, by which micro-optical systems can be implemented. Micro-optical components such as optical modulators and micro-lenses, etc., corresponding to a micro-optical system, is selected for application in telecommunication devices, displays, and recording devices, due to such advantages as quick response time, low level of loss, and convenience in layering and digitalizing.
The optical modulator is a circuit or device which loads signals on a beam of light (optical modulation) when the transmission medium is optical fiber or free space in the optical frequency range. The optical modulator can be divided mainly into a direct type, which directly controls the on/off state of light, and an indirect type, which uses reflection and diffraction, where the indirect type may further be divided into an electrostatic type and a piezoelectric type according to how it is operated.
Regardless of its operation type, the indirect type optical modulator performs optical modulation by means of interference occurring due to the differences in paths between lights reflected or diffracted on different surfaces. In particular, a piezoelectric type optical modulator generates differences in paths of reflected light using the operating power of piezoelectric elements, which contract and expand according to a predetermined voltage supplied to the optical modulator (refer to descriptions for FIGS. 5 and 6). Thus, in a piezoelectric type optical modulator, the piezoelectric elements play an especially important role in implementing its light diffraction properties.
However, in prior art, extended periods of use of the optical modulator may cause separation at the interface (see portion of FIG. 1 indicated by dotted lines) between a piezoelectric element and the LTO layer (low temperature oxide layer) formed at the lower portion (i.e. the lower portion of the lower electrode) of the piezoelectric element, to create defects in the optical modulator (see portion of FIG. 2 indicated by dotted lines). This is because the conventional Ti thin film, stacked as a junction layer for adhesion between the LTO layer and the lower electrode, becomes degraded with extended periods of use. The ‘c’ portion within the portion indicated by dotted lines in FIG. 2 show a piezoelectric element that has not been separated from the LTO layer (the entire ‘c’ portion of FIG. 2 displays a dark yellow color), while the ‘a’ and ‘b’ portions within the portion indicated by dotted lines in FIG. 2 shows parts of the piezoelectric elements separated from the LTO layers (parts of the ‘a’ and ‘b’ portions of FIG. 2 display a white color). Here, portions ‘a’ through ‘c’ show the positions where piezoelectric elements are formed.
Also, the conventional Ti thin film has a tendency of becoming oxidized by the oxygen diffused during the high-temperature RTA (rapid thermal annealing) process for stacking a piezoelectric layer, performed after the process for forming the lower electrode. Thus, as the Ti thin film is oxidized by the diffused oxygen, it is more easily separated from the LTO layer.
The degrading of the Ti thin film as described above causes a degrading of the overall piezoelectric element, and is consequently detrimental to the light diffraction property and reliability of the optical modulator.